Our own home-made pH paper.
We soaked blotting paper (any thickish absorbent paper will do) in a solution of turmeric in alcohol. (Rubbing alcohol is recommended - we used meths, cos that's what we had).
Dipping the strip into acid substances, such as vinegar should turn the strip a deep orange/red. If you hold the strip over an open bottle of ammonia (we didn't have any) it should then turn pale again.
Krampf has a good video on this (only available to members, tho') here at thehappyscientist